One of its recent photos captured the moment Mars' small, asteroid-sized moon Phobos passed in front of the sun.

The transit of Phobos saw it obscure about 5% of the sun on 13 September. Observing it allows Nasa's scientists to learn about Mars' internal structure, because the moon's orbit causes small changes to the shape of the planet which can be observed through changes in the moon's orbit, and observed.

Nasa explained on its website:

"Mars has two small, asteroid-sized moons named Phobos and Deimos. From the point of view of the rover, located near the equator of Mars, these moons occasionally pass in front of, or "transit," the disk of the sun.

These transit events are the Martian equivalent of partial solar eclipses on Earth because the outline of the moons does not completely cover the sun (in contrast, Earth's moon does block the entire sun during a total solar eclipse)."

Meanwhile the Curiosity rover is about to begin investigating a strange, football-sized rock which Nasa said will help configure its instruments.